1. Field of the Invention
This invention relates, in general, to mercury vapor pressure control materials used for a mercury vapor discharge lamp. In particular, the invention relates to an amalgam sealed in a bulb to control the mercury vapor pressure in the bulb within a prescribed range during the operation. The invention also relates to a low mercury vapor pressure discharge lamp, using the above-described amalgam, which operates under a medium bulb surface temperature, e.g., 60.degree. C. or 70.degree. C.
2. Description of the Related Art
In general, a low mercury vapor pressure discharge lamp, e.g., straight type fluorescent lamp, is provided with pure mercury. The mercury vapor pressure in the bulb is maintained at substantially 6.0.times.10.sup.-3 torr when the bulb surface temperature is 40.degree. C. Under such a mercury vapor pressure described above, the fluorescent lamp can operate at its desirable characteristics.
In recent years, small sized fluorescent lamps, called ball-type fluorescent lamps, have been developed. In one of the ball-type fluorescent lamps (hereafter referred to as a high temperature ball-type lamp), the bulb is bent and is housed in a small sized hermetic globe together with a ballast. In this type of the lamp, since heat radiated from the bulb and the ballast does not readily escape from the globe, the bulb surface temperature of the lamp increases above 90.degree. C. during the operation. In this case, the luminous flux of the lamp decreases if the mercury vapor pressure in the bulb increases excessively. To control the mercury vapor pressure within a prescribed range, an amalgam, such as, e.g., bismuth (Bi)-indium (In)-amalgam, etc., is sealed in the bulb. Such amalgam controls the mercury vapor pressure in the bulb at a desirable value when the bulb surface temperature is about 90.degree. C.
On the other hand, another ball-type fluorescent lamp in which the ballast is attached to the outside of the globe also has been developed, as a low mercury vapor pressure discharge lamp. In this type of lamp (hereafter referred to as a low temperature ball-type lamp), a small fluorescent lamp which has a U-shaped discharge pass or an H-shaped discharge pass and operates under a high wall loading above 500 W/m.sup.2, is used.
In such a low mercury vapor pressure discharge lamp described above, the bulb surface temperature is low, as compared with the above-described high temperature ball-type fluorescent lamp in which the amalgam is sealed. However, the bulb surface temperature of the above-described low temperature ball-type lamp is relatively high, as compared with the conventional straight-type fluorescent lamp in which pure mercury is sealed. In this case, a desirable luminous flux of the low temperature ball-type lamp is not achieved even if the above-described bismuth-indium-amalgam is sealed in the bulb to control the mercury vapor pressure.
This is because the bulb surface temperature of the low temperature ball-type lamp is low, as described above. Therefore, such amalgams described above do not operate efficiently. In this case also, if pure mercury is used rather than the above-described amalgams, the mercury vapor pressure in the low temperature ball-type lamp increases to an excessive level, and thus, a desirable luminous flux also is not achieved.